Permanent magnet synchronous motor (hereinafter, simply “motor”, unless otherwise requires particular distinctions) has been known to be a highly efficient motor. As compared to induction motors conventionally used in various fields, the permanent magnet synchronous motor does not require any excitation current because a magnetic flux is established by a permanent magnet incorporated in a rotor, and secondary copper loss does not occur because any current does not flow in the rotor unlike induction motors. While electric vehicles have conventionally used induction motors, application of the permanent magnet synchronous motor to them has been studied in recent years to achieve efficiency improvements.
In electric vehicles, a driving controller for an AC motor and a motor are incorporated in each of plural combined vehicles. Even when a short-circuit fault occurs in a driving controller for an AC motor in a certain vehicle while the vehicle is running, the electric vehicle can continue to run by other driving controllers for AC motors and motors that are normally operating. The motor connected to the defective driving controller for an AC motor continues to be driven by wheels. Consequently, a short-circuit current caused by an induced voltage of the motor continues to flow in a part of the driving controller for an AC motor where the short-circuit fault has occurred (a short-circuited part).
When such a state is left as it is, the defective part of the driving controller for an AC motor can be further extended by heat generated by the short-circuit current, and the defective part or the motor can generate heat or burnout, which is not preferable.
To deal with such cases, for example, Patent Document 1 discloses a method in which a motor cut-off contactor provided to serve as a motor side switch electrically disconnects an inverter from a motor. A control unit controls the contactor to open for electrically disconnecting the inverter from the motor when a fault in the inverter is detected. Thus, the inverter is not further damaged when the fault occurs in the inverter within a driving controller for an AC motor controlling to drive a permanent magnet synchronous motor while an electric vehicle is running.    Patent Document 1: Japanese Patent Application Laid-open No. H8-182105
As generally known, because a current zero point is generated in an alternating current with a sinusoidal waveform for every half period of a current waveform, the current can be interrupted utilizing the current zero point. The motor cut-off contactor disclosed in Patent Document 1 mentioned above is a contactor that interrupts an alternating current utilizing the current zero point to interrupt the current. Examples of the contactor that interrupts an alternating current generally include a vacuum contactor, to which a system of interrupting a current at current zero points is applied.